Ion sources of the prior art generally use a filament-heated cathode to strike a low voltage discharge in a gas introduced into the source. Solid deposits from the gas contaminate the chamber and occlude the ion aperture. The filaments have a short life and the gases are often toxic. Toxic gases require special handling and pumping. It would be desirable to have an ion source in which there is no filament to burn out and in which the source material can be incorporated as a solid to eliminate the problems of toxic gases, and which is self-cleaning.
A planar magnetron has a plane cathode sheet over which there is a re-entrant magnetic field pattern formed by lines of north and south poles placed under the sheet. (See Waits, "Planar Magnetron Sputtering", JVST 15 (2), p. 179, March/April 1978.) Typically the north pole is a ring concentric with a south pole ring parallel to the plane of the cathode. The magnetic field which loops above the cathode forms a "racetrack" which traps an intense donut-shaped discharge, anchored to the cathode, analogous to the negative glow in a discharge between parallel plates when B=0. Primary electrons are released from the cathode by ion bombardment and give up their energy in the negative glow. A weaker glow which pervades the external space, like a positive column, connects the negative glow of the donut to an anode of arbitrary location. Above a pressure of about 1 mTorr, the planar magnetron behaves approximately as a glow discharge with a fixed voltage of about 400 volts in argon and unlimited current density. Most of the cathode current is carried by ions. Substantially all of the discharge voltage appears across a thin sheath between the negative glow and the cathode. The thickness of the cathode sheath is believed to be governed by Child's law for space charge limited emission. Thus, for a current density J, discharge voltage V, and sheath thickness x ##EQU1## amps per unit area (in units of x.sup.2), for singly charge ions of atomic weight M. If V=400 volts, M=40 (argon) and x=1 mm, we get J=7 ma/cm.sup.2. Experimentally, a value of J=100 ma/cm.sup.2 is easy to obtain, and we expect a sheath thickness of a fraction of a millimeter. (See Thornton, "Magnetron Sputtering", JVST 15 (2), p. 171, March/April 1978; Green, "Intense Ion Beams", Rep. Prog. Phys. 37, p. 1257, 1974; Alton, "Aspects of the Physics . . . of Heavy Ion Sources", Nucl. Inst. Meth. 189 (15), p. 37, 1981.)
The planar magnetron is an important industrial device which has been subjected to little research; it is known to be stable and well-behaved at pressures of a few milliTorr, with capability for high discharge current density limited only by power dissipation.
In the publication of Heisig et al, "High Rate Sputtering With a Torous Plasmatron", Proc. 7th Intl. Sym. Electron and Ion Beam Science and Technology, Wash., D.C. 1976, Electrochemical Society, Princeton, NJ, 1976, pp. 129-141, a sputter deposition source is described using a cylindrical inverted configuration.
U.S. Pat. No. 4,542,321 to Singh et al discloses an inverted magnetron ion source in which the lines of magnetic flux extend through a cylinder cathode from one end to the other. This device does not concentrate the plasma enough to form an intense ion beam.